1. Field of the Invention
The present invention generally relates to hydrostatic transmissions and transaxles and, more particularly, is concerned with securing the motor track ring within the hydrostatic transaxle.
2. Description of the Prior Art
Hydrostatic transmissions transmit rotary mechanical motion, typically from an internal combustion engine, to fluid motion, typically oil, then back to rotary mechanical motion to rotate a drive axle in order to drive a vehicle such as a lawn and garden tractor. The hydrostatic transmission regulates or controls the output rotary mechanical motion such that varying output speeds in the forward and reverse directions are possible with a single speed input rotary mechanical motion. Within a hydrostatic transmission of the radial piston type, a cylinder unit having a pump and motor rotates on a fixed pintle with pistons positioned within the cylinders and attached to slippers mounted in an expander band so that as the cylinder unit rotates, the pistons drive or are driven by the slippers which engage the surrounding eccentric annular track ring of the pump and motor. The pistons of the pump create a pressurized fluid flow that drives the motor pistons which rotate an output shaft. The transmission ratio is therefore directly proportional to the eccentricity of the track ring of the pump relative to the fixed pintle and the motor track ring. This type of hydrostatic transmission is shown in U.S. Pat. No. 4,979,583 entitled Variable Speed Transaxle, issued Dec. 25, 1990, and pending application Ser. No. 07/535,462 entitled Variable Speed Transaxle filed Jun. 8, 1990 both of which are specifically incorporated by reference herein.
The motor track ring, however, does not need to change its eccentricity and is therefore fixed within the transmission. Prior art methods of fixing the motor track ring within the transmission casing have included seating the motor track ring by extending the pivot pin of the pump track ring through the motor track ring. This however requires additional parts to connect the pivot pin to the motor track ring and also enlarges the inboard side of the hydrostatic portion of the unit. This is disadvantageous in that enlarging the inboard side of the hydrostatic portion necessitates that the output gearing from the hydrostatic unit be moved axially further out, thereby lengthening the overall dimension of the transaxle. In addition, this requires that the differential be axially shifted in the same direction, thus shortening one of the axles. In shortening one of the axles, that axle is less able to withstand transverse loading. It is thus advantageous for both axles to be as long as possible in order to provide a better bearing support.